Fracturing of an underground hydrocarbon formation along a wellbore extending through the formation by injection of pressurized fluids into the formation via the wellbore have been used for a number of years.
Specifically, injection of pressurized fluids in hydrocarbon formations at pressures above formation dilation pressures has been used in the past to provide fractures and fissures in rock surrounding a wellbore, to thereby stimulate a reservoir to release oil therein by providing channels within the fractured rock which oil the formation may then flow through to then be collected.
The fracturing fluid which is provided under pressure may be a non-compressible fluid such as water, and/or further containing proppants and/or hydrocarbon diluents for the purpose of not only creating fissures in the rock but for further propping and maintaining the fissures in an open position to allow oil to flow through and/or reduce the viscosity of the oil and cause it to more readily flow through created fissures in the rock.
Disadvantageously, however, in hydrocarbon formations where the characteristics of the formation may not be completely understood or known at all locations in the formation, injection of pressurized fluids along an entire length of a wellbore may inadvertently inject liquids into regions of the formation where the porosity of the formation at certain regions may already be such that such is not needed, or are locations containing relatively less hydrocarbons, which in either case such is wasteful of the injected fluid. This is particularly of concern in instances around the world where water, which is typically a principal component of the injected fluid, is scarce, difficult to obtain, or not available.
Also disadvantageously, hydrocarbon reservoirs often possess regions of high water content. Fracturing along an entirety of the length of a wellbore and thus in all regions of a formation bounding a wellbore will typically undesirably result in fracturing of rock in one or more high water content regions. Such fracturing thereby allows water therein to more easily flow out of such regions and into the wellbore, and conversely allows oil to flow into these regions when water has vacated, thereby detrimentally affecting recovery of hydrocarbons through the wellbore.
Accordingly, for the above reasons, indiscriminate fracturing along a wellbore, without having intimate knowledge of the in situ geology and in particular the porosity of the formation directly in the region of the wellbore often leads to reduced recovery from the formation via that wellbore that would otherwise be the case if the porosity and “tightness” of the oil at various discrete locations along the wellbore was otherwise known.
Accordingly, a real need exists in the oil recovery industry of an in-situ method to allow reservoir and production engineers to better understand, for a particular reservoir, the geology and porosity of the formation in regions bordering the wellbore, and in particular which regions of a formation immediately adjacent such wellbore may be “tight” and thus where oil is potentially trapped and which are in need of stimulation through fracturing and/or injection of proppants and/or diluents, as distinguished from other regions of the formation along a wellbore which are not as “tight” and for which injection of fluids into such regions may not produce as much benefit and/or stimulation thereof which may prove detrimental to oil recovery.
As regards downhole tools for injecting fluid under high pressures as commonly used for conducting fracturing operations, such tools have likewise been known and used for a number of years. More recently, however, downhole tools have been developed which provide high pressure cyclic pressure surges, instead of a single high pressure, which is more effective in providing stimulation as it avoids constant high pressure application to the formation which might otherwise displace oil from the region of the wellbore and/or negatively affect the created fissures.
Examples of recent downhole tools which provide pulses of pressurized fluid at pressures in excess of formation dilation pressures to propagate pressure waves through a formation are tools/valves such as those described in U.S. Pat. No. 7,806,184 entitled “Fluid Operated Well Tool” and U.S. Pat. No. 7,405,998 entitled “Method and Apparatus for Generating Fluid Pressure Pulses”, each of said patents commonly assigned to one of the a co-assignees of the within invention.